1. Field of the Invention
The invention relates to a system and method for sharply illuminating a predetermined region of a reticle during exposure.
2. Background Art
A relay objective (e.g., lens) or a reticle edge masking assembly (REMA) objective (e.g., lens) is an objective that images an intermediate plane onto a plane of a reticle. The reticle supports a mask for lithography. By using the relay lens, the region illuminated on the reticle is sharply defined. Usually, the reticle masking device is assembled with adjustable edges. Conventionally, the relay lens is utilized in microlithographic exposure systems, steppers, or scanners, although the relay lens can be used in other optical systems. Diaphragm edges, which lie in the object plane of the relay lens, must be imaged precisely onto the reticle plane. An exposed corrected pupillary intermediate image is often desired because then, at the location of the intermediate image, further diaphragms and the like can be mounted, for example, to mask parts of an alignment system.
Typically, relay lens systems have very complicated structures (e.g., from about 7 to 10 lens elements). These optical systems have high Numerical Aperture (NA) (e.g., about 0.6 to 0.7). Basically, these systems include of three parts: a front portion (this part decreases NA), an intermediate portion (for pupil aberration and pupil shape correction) and a field portion (to create essential field size on the reticle). Following the relay lens system in a lithography tool is a projection objective, which normally operates in reduction and can include an inner-lying pupillary plane for non-telecentric input. A wafer follows in an image plane. An additional task of the relay lens system is to correct telecentricity on the reticle.
Conventional systems have a plurality of the optical elements in each of the above-mentioned front, intermediate, and field parts. The front part has 3-4 lenses, the intermediate part has 2-4 lenses, and the field part has 2-4 lenses. Conventional relay lens systems have several disadvantages because of the plurality of the optical elements in each part, these are: (1) a large volume of CaF2 is needed for 157 nm lithography systems, which is quite costly; (2) there is low transmission caused by not only glass absorption, but also reflectance on each lens surface; (3) there are alignment difficulties  because too many lenses are required to be aligned, and (4) there is high cost due partially to the large volume of Ca F2 required. These high costs are then passed on to purchasers of the systems, which can make purchasing additional or newer systems prohibitive.
What is needed is a relay lens that is a simpler, a less complex design, and that reduces costs involved in manufacturing, and thus in customer costs, which can all be achieved by reducing optical elements in the relay lens.